iPhone restarts asking for my 6 digit code, but I only have a 4 digit code

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I have given my son my old iPhone 6 since when I got the new 7 pus. Here is the problem:

He will be in the middle of a game and then all the sudden the game shuts down and the screen just turns into different colors. So, he turns it off and back on and it asks for a 6 digit code when he only has a 4 digit code set.

This is the second time it happened. The first time we had to wipe the phone back to new and re set it up, then we did a restore from the last backup.

The second time it happened we were getting ready to go through the process again and all the sudden the touch ID worked and it came back on.

Any thoughts?

Originally started using iOS 10 and is now upgraded to 10.1.1 when this second instant occured